tests/extmod: Add test for ujson.load().

tests/extmod/ujson_load.py

+try:
+    from uio import StringIO
+    import ujson as json
+except:
+    from io import StringIO
+    import json
+
+print(json.load(StringIO('null')))
+print(json.load(StringIO('"abc\\u0064e"')))
+print(json.load(StringIO('[false, true, 1, -2]')))
+print(json.load(StringIO('{"a":true}')))